Calving
When Will She Calve? Tips for producers on predicting a calving date. by Heather Smith Thomas
C
ows generally calve nine months and seven days after conception. The average length of pregnancy is approximately 283 days, but this figure is just an average and very few cows calve on their estimated due date. Gestation length is partly a matter of genetics. Some breeds and some family lines within breeds tend to have varying gestation lengths, and this is a factor in calving ease. Cattle with shorter gestation lengths generally give birth to smaller calves since the fetus is growing fastest at the end of gestation. A 278-day gestation usually results in a smaller, lighter calf than a 288-day gestation. One study showed that for each extra day of gestation, there is approximately one pound of increase in the size of the calf.
Controlling gestation length One reason bull calves are usually larger and heavier than heifer calves is they tend to be carried longer — partly due to hormonal factors. A cow that calves a few days before her estimated due date often gives birth to a heifer, whereas a cow that goes overdue often has a bull calf.
To increase calving ease, a producer can select bulls with a shorter gestation length to use on the cow herd.
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| January 2018
Buddy Westphal is a seedstock producer in Montana with a herd of 600 cows. He says he pulls less than 10 calves per year, but 90% of the time when he assists with the birth it’s a bull calf. “They usually go a few days past the due date and tend to be a little bigger,” Westphal explains. Actual due date can vary depending on gestation length of certain individuals. Some cows consistently tend to calve earlier or later than their projected due date. In contrast, some bulls sire calves that are regularly born earlier or later. Fetal development and rate of maturation is influenced by the genetics of both the sire and the dam. One reason a low-birthweight bull tends to sire smaller calves is his calves inherit a shorter gestation length than average. If both the sire and dam tend to have short gestation lengths, the calf will be born earlier. If either the sire or the dam has genetics for a longer gestation, the results may be mixed depending on which trait is inherited by the calf. “Birthweight of the cow has an influence on the birthweight of her calf no matter what you breed her to,” Westphal explains. “The calf may be larger than you hoped.” He has been interested in gestation length for many years — particularly as it applies to calving ease, more time for the cow to recover from calving and to breed back on time. When he started his purebred operation 45 years ago after attending Colorado State University (CSU), the cows on the ranch he bought had significant calving problems. The bulls he was using sired big calves that consistently went a week or two past their estimated due dates. “My professors at CSU, including Dr. Wiltbank who did a lot of research in Miles City with Dr. Bellows, taught us that a cow had to have a calf every 365 days to be profitable,” Westphal says. “It doesn’t matter whether you calve in spring or fall, she needs to calve on the same schedule every year.” Otherwise she falls behind with a calf that doesn’t fit the producer’s program, and she may eventually end up open. “When a cow is calving 10 days or two weeks late, especially if she has a big calf and a hard birth, she’s losing that amount of time to recover and rebreed on schedule,” he explains. “She’ll calve two to three weeks later the next year, and her calf will be 20 lb. lighter than the others.” After a few years of that cycle, she will inevitably come up open. As a result, Westphal looks for bulls that sire calves with a shorter gestation. Using those bulls enables him to get his cows bred back quicker. “I went to the Canadian Conception to Consumer test, where they verified bulls of many breeds through their testing program, and they had exact gestation lengths on all of them,” he notes. “I was looking for bulls that had a 283-day or less average gestation length. I found some bulls that fit into that category and started using those. Instead of siring a 120-pound calf, they’d sire an 80-pound calf, and it would be born a lot easier.” Thus, the calving problems were reduced, and that reduction solved the dilemma of getting the cows bred back on time. Forty years later, his calving season is much easier and 75% of his calves are coming from the Hereford.org